The present invention relates, in general, to product storage tanks having floating roofs, and, more particularly, to secondary seals used in such tanks.
Products, such as petroleum products, and the like, are frequently stored in tanks which have floating roofs.
These storage tanks utilize seals to close the annulus defined between the roof and the tank shell. Frequently, there are two seals used: a primary seal which is the major closure means between the floating roof and the tank shell to prevent product loss by evaporation and which customarily contacts the stored product, and a secondary seal which functions to protect the primary seal from the deteriorating effects of sun and acts as a barrier between the primary seal and wind gusts. The secondary seal also serves as the initial barrier in preventing foreign material from entering the tank via the annular gap between the roof and the tank shell. The secondary seal also prevents evaporative product loss, especially by protecting the product from wind gusts which form one of the principal causes of evaporative losses from open top floaters.
The secondary seal thus protects the primary seal, while also functioning to assist the primary seal in protecting the product, but does not contact the product. An analogy can be made to a boot and a shoe, where the secondary seal is analogous to a boot which both protects the shoe and assists the shoe in protecting a foot.
The assignee of the present invention also owns a further patent on a secondary seal design. This patented seal functions well on smooth walled, gasoline storage tanks.
However, there are several drawbacks to this patented seal when it is used on waxy crude storage tanks. When a floating roof is cycled in crude service, the heavy wax adheres to the tank wall above the seal. As the sun warms the wall, this crude melts and flows downwardly over the secondary seal and onto the roof. Besides emissions and safety concerns, this crude can flow into and foul drainage systems. At night, this build-up of crude on the tank wall solidifies around the secondary seal, thus freezing the seal in place. When the roof is again cycled, the secondary seal is no longer free to move and is subjected to loads that can buckle the seal or tear it and pull it apart. Such loading can also damage the roof and the wall.
Prior secondary seals do not adequately protect the primary seal and often extend above the roof, thereby inhibiting use of the entire height of a tank for storage. The amount of lost storage volume increases proportionally as the diameter of the storage tank increases. Furthermore, known secondary seals do not have a surface which contacts the tank wall with an area sufficient to maximize control of emissions and wind gusts.
Furthermore, known secondary seals will not effect a seal if the annulus between the roof and the tank wall exceeds the reach of these known seals by any amount.